A technique for producing geometrically accurate magnetic resonance images (MRIs) with undistorted intensity in the face of high levels of static field inhomogeneity arising from either source is presented. The technique requires the acquisition of two images of the same object with altered gradients. On the basis of a knowledge of these gradients it employs an automatic postprocessing step that exploits some invariant characteristics of the distortions to produce a rectified image from the two acquired images. No phantom imaging is involved and no operator interaction is required. The technique is theoretically justified and compared to other techniques, and experimental results that show that the technique works are presented. The improved accuracy in geometry and intensity may improve reliability of stereotactic surgery, may enhance the feasibility of both clinical and industrial imaging via external fields, and may increase the resolution of microscopic imaging.

A general and straightforward model was developed for the design of passively -switched lasers. With the secondthreshold criterion and using a numerically fitting procedure, the output pulse energy was expressed as an analytical function of the initial transmission of the saturable absorber and the reflectivity of the output coupler. An analytical expression for the optimal output reflectivity was also obtained for maximizing the output pulse energy of a passively -switched laser with a given initial transmission of the saturable absorber. Excellent agreement was studied between the present results and detailed theoretical computations. A Nd:YAG laser with Cr 4+ :YAG as a saturable absorber was performed to illustrate the use of the present model. Index Terms-Passively -switched laser, saturable absorber, solid-state laser, Cr 4+ :YAG.

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